With the increased diversity of IT devices and the like, recent years have seen increasing needs for compact surface-emitting devices consuming less power. As one of such surface-emitting devices, an electroluminescent device (hereinafter, abbreviated as EL device) is drawing attention.
Such EL devices are generally classified into an inorganic EL device and an organic EL device by the material used therein.
The inorganic EL device is generally adapted to emit light as follows. A high electric field is applied to a luminous portion whereby electrons are accelerated in the high electric field to impinge upon a luminescence center, which is excited to emit light. On the other hand, the organic EL device is adapted to emit light as follows. Electrons and holes are injected into a luminous layer from an electron injection electrode and a hole injection electrode, respectively. The injected electrons and holes are combined together in the luminous layer so as to bring an organic material into an excited state. The organic material emits light when the material in the excited state returns to a ground state. The organic EL device has an advantage that this device can be driven with a lower voltage than the inorganic EL device.
In the case of the organic EL device, a luminous element capable of emitting light of a desired color can be obtained by selecting a suitable luminous material. Further, the organic EL device is also adapted to emit white light by using a suitable combination of luminous materials. It is also is anticipated to use the organic EL device as a backlight of a liquid crystal display device and the like.
In a case where the organic EL device is used as the backlight of the liquid crystal display device or the like, the EL device is generally required of front brightness on the order of 2000 to 4000 cd/m2.
However, when the surface-emitting device such as the above EL device is activated to emit light, the emitted light beams travel in various directions. Hence, many of the beams undergo total internal reflection at an output surface and the like of the surface-emitting device so as to be trapped in the surface-emitting device. It is therefore difficult for the device to achieve a sufficient front brightness. Particularly, the organic EL device has a problem that the device can only provide brightness on the order of 1000 to 1500 cd/m2 if it is designed to ensure a sufficient light emission life.
In the art, a surface light emitter has been proposed which is provided with a light control sheet as follows in order to increase the front brightness by extracting light trapped in the surface-emitting device, such as the organic EL device, when the surface-emitting device is activated to emit light (see Japanese Unexamined Patent Publication No. 2000-148032). The light control sheet is formed with a plurality of projections and is tightly bonded to the output surface of the surface-emitting device at surfaces of distal ends of the projections thereof.
However, even though the light control sheet formed with the plural projections is tightly bonded to the output surface of the surface-emitting device at the surfaces of the distal ends of the projections thereof, as described above, the light reflection and the like vary greatly depending upon the configuration or arrangement of the projections of the light control sheet. The problem of inability to achieve the sufficient increase in the front brightness is still unsolved. Specifically, the prior-art surface light emitter exhibits the maximum brightness on the order of 1500 to 2000 cd/m2 and achieves only about a 1.5-fold increase in the front brightness in maximum. It is still difficult to obtain a sufficient brightness.